A high-density carbon material is excellent in the heat resistance and chemical stability, and electrically conductive and therefore, is widely used as a structural member or an electrical or electronic material. In addition, since carbon exhibits a capability of reducing many metal oxides at a high temperature, the carbon material is also used as a metal reducing agent for refining titanium or the like.
As the method for producing a high-density carbon material, there is known a method where an aggregate component having a high carbon content and undergoing carbonization without melting, such as coke, and a binder component having thermoplastic properties and capable of binding aggregates with each other and moreover, being carbonized, such as coal tar pitch, are mixed and the mixture is formed, followed by subjecting to a heating (carbonization) treatment at a high temperature, thereby achieving the carbonization thereof. This method has a problem that the actual carbon ratio of the binder component is low, and one carbonization treatment allows for the presence of a void, leading to a carbon material having a small density. Accordingly, the carbon material after the carbonization treatment is impregnated with the binder component and again treated for carbonization and is necessary to be densified by repeating such a process many times. In turn, the production process of a high-density carbon material becomes cumbersome, and the production also takes a long period of time, as a result, the productivity is low and the high-density carbon material is expensive.
As the method for producing a high-density carbon material without use of a binder, a high-density carbon material using a carbon raw material having self-sinterability has been proposed. The self-sinterability is such a property that the material can be formed without addition of a binder component and when it is heat-treated, it is carbonized while maintaining the shape.
As a representative example of the carbon material having self-sinterability, a mesocarbon microbead is known.
Recently, from the standpoint of enhancing the quality in various applications, the content of impurities (so-called ash) other than carbon in the carbon material is required to be small, but since conventional carbon raw materials have a large impurity content, it has been difficult to provide a high-purity carbon material.
As regards a carbon raw material having a small impurity content, use of ashless coal containing substantially free of ash is being studied (for example, Patent Document 1). However, ashless coal has high thermal fluidity and has a property of melting at 200 to 300° C. irrespective of the grade of the raw material coal. In addition, ashless coal has a property of expanding when it is heated at around 400° C. Therefore, when a formed body obtained using ashless coal is carbonized, vigorous foaming occurs due to high-temperature heating to involve expansion, giving rise to a problem that cracking or chipping is produced in the carbon material, powdering is caused to fail in maintaining the shape of the formed body, or the carbon material becomes porous and is decreased in the density.
To solve such a problem, the present inventors have proposed a technique for modification of ashless coal (Patent Document 2). In this technique, the volatile matter content is adjusted to fall in a predetermined range by heating the ashless coal, thereby making it possible to provide a high-purity carbon material that is enhanced in the self-sinterability, kept from expansion even when treated for carbonization, free of cracking, chipping or powdering, and capable of maintaining the shape formed.